To determine the performance of a synchronous optical network, such as a DWDM-SDH/SONET network, it is known to measure the signal quality of optical links during operation by observing the amplitude and phase distribution of the optical signal and derive an estimate of the bit error rate therefrom; DWDM=dense wavelength division multiplexing, SDH=synchronous digital hierarchy, SONET=synchronous optical network. To this end, a Q-factor measurement is performed, which directly reflects the quality of the optical signal; Q stands for the error integral. Q is obtained by observing the amplitude and phase characteristic of the analog signal at the electrical level. The method is based on nonuniform sampling to simulate a poor signal-to-noise ratio and an increased bit error rate. The Q factor, which corresponds to the conditions with optimum sampling, can than be determined using statistical methods. The principle of the Q-factor meter is based on random sampling of the eye pattern. In a two-way detector, one comparator operates as a center sampler with a fixed amplitude threshold and fixed phased position, while another comparator operates as a measuring sampler and can vary these two parameters. The outputs of the two comparators are compared by an exclusive-OR gate, and differences are counted as bit errors. The method is limited to synchronous, continuous-time signals with a stable, previously known sampling clock phase.